A slit lamp with laser capability for ophthalmic surgery is disclosed in copending application Ser. No. 417,689, filed Sept. 13, 1982 and entitled "Instrument for Ophthalmic Laser Surgery". The slit lamp of that application is mounted on an x-y table so that the whole slit lamp assembly can be moved toward and away from the patient's eye as well as sideways by moving the x-y table in the desired directions. The movement capability in the y direction allows the surgeon to focus the laser beam at various depths within the patient's eye. The focusing is done with the aid of a benign aiming laser and is normally done empirically by the surgeon while observing the patient's eye.
For greater accuracy and improved efficiency, it would be desirable for the surgeon to be able to position the table at a particular point or along a particular line repetitively and without the need for careful empirical refocusing, as well as to be able to continuously observe an accurate measurement of the table's movement (and other relevant data) while observing the patient's eye.